Present commercial copiers and printers which create duplex output generally fall into one of two categories. In a double pass system, images are transferred to one side of a series of copy sheets, those images are fused and the copy sheets are placed in a duplex tray. From the duplex tray they are refed to the transfer station to receive images for the other side of the copy sheet, these images also are fused and sent to an output hopper. See, for example, U.S. Pat. Nos. 3,630,607 and 4,099,150.
In a single pass system, the copy sheet is fed to a transfer station to receive an image on one side and then is turned over and fed back to either the same transfer station or a second transfer station to receive an image on the other side. U.S. Pat. No. 3,672,765 shows an early example of this approach in which two transfer stations and two fusers are used, the first image being fused before the copy sheet is turned over for feeding to the second transfer station. The second fuser is used to fuse the second image before the copy sheet goes to the output hopper.
The Kodak Ektaprint 250 copier introduced in 1982 provided a substantial advance in the single pass form of duplexing. In that copier a turnaround mechanism is positioned between the first and second transfer stations and a copy sheet transport mechanism is positioned after the second transfer station, neither of which disturbs either of the powder images, permitting fusing of both images simultaneously after the second transfer. This system has a number of advantages over the two-pass system. The paper path is considerably shorter, paper jam clearance is easier, access time to the first sheet is greatly reduced, and the order of imaging in automatic collation systems is more straight forward. This latter advantage is especially useful when the system is used as part of a non-impact printer because data fed to a printer is generally in page sequential order, an order which printers using a duplex tray cannot conveniently use. This system also eliminates significant disadvantages of systems in which the images are separately fused. That is, the two images in such latter system get different amounts of fusing and the second transfer system must operate with a warmed copy sheet which may result in image voids from wrinkles and the like. It also saves the expense of an additional fuser often used in such systems.
Present electrostatographic color systems create separate toner images of different color and transfer them in registry to a single copy sheet. The separate toner images can be created on the same or different electrophotosensitive members.
Examples of many disclosures showing creation of the separate toner images on different electrophotosensitive members are U.S. Pat. No. 4,120,577 and U.S. Pat. No. 4,162,843. In these references a sheet is fed past three or four electrophotosensitive members each with a different color toner image and the images are transferred in registry to the sheet.
U.S. Pat. No. 4,477,176 shows an adaptation of the single-pass duplexing structure to multicolor duplex reproduction. According to FIG. 3 of that patent, color separation images for two multicolor images are formed in order on a single electrophotosensitive web. A copy sheet is fed to first and second transfer rollers, each of which have means for holding the copy sheet through at least two revolutions bringing it into contact with consecutive images to receive them in registration. The first transfer drum is reversible for inverting the copy sheet after it has received two or more images in registry on one side. The copy sheet is then fed through a fuser, is tacked to the second transfer drum, receives images in registry on the other side, is fed then to a second fuser and ultimately to an output hopper.
U.S. Pat. Nos. 3,838,919; 4,515,460; 4,652,115; 4,712,906 and 4,714,939 are examples of the use of intermediate drums or webs to receive a plurality of toner images in registration from which webs or drums the registered toner images are transferred in one step to a receiving sheet. These devices have the advantage of more precise registration than devices in which a copy sheet must be held securely as it is circulated through transfer relation with the images. The latter reference shows a duplex version, one side of which can be multicolor, in which an intermediate web is used for the second side image.